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How to migrate data using the expand and contract pattern

Introduction​

When making changes to your database schema in production, it's crucial to ensure data consistency and avoid downtime. This guide shows you how to use the expand and contract pattern to safely migrate data between columns. We'll walk through a practical example of replacing a boolean field with an enum field while preserving existing data.

Prerequisites​

Before starting this guide, make sure you have:

  • Node.js installed (version 18 or higher)
  • A Prisma ORM project with an existing schema
  • A supported database (PostgreSQL, MySQL, SQLite, SQL Server, etc.)
  • Access to both development and production databases
  • Basic understanding of Git branching
  • Basic familiarity with TypeScript

1. Set up your environment​

1.1. Review initial schema​

Start with a basic schema containing a Post model:

generator client {
  provider = "prisma-client-js"
}

datasource db {
  provider = "postgresql"
  url      = env("DATABASE_URL")
}

model Post {
  id        Int     @id @default(autoincrement())
  title     String
  content   String?
  published Boolean @default(false)
}

1.2. Create a development branch​

Create a new branch for your changes:

git checkout -b create-status-field

2. Expand the schema​

2.1. Add new column​

Update your schema to add the new Status enum and field:

model Post {
  id        Int      @id @default(autoincrement())
  title     String
  content   String?
  published Boolean? @default(false)
  status    Status   @default(Unknown)
}

enum Status {
  Unknown
  Draft
  InProgress
  InReview
  Published
}

2.2. Create migration​

Generate the migration:

npx prisma migrate dev --name add-status-column

3. Migrate the data​

3.1. Create migration script​

Create a new TypeScript file for the data migration:

import { PrismaClient } from '@prisma/client'

const prisma = new PrismaClient()

async function main() {
  await prisma.$transaction(async (tx) => {
    const posts = await tx.post.findMany()
    for (const post of posts) {
      await tx.post.update({
        where: { id: post.id },
        data: {
          status: post.published ? 'Published' : 'Unknown',
        },
      })
    }
  })
}

main()
  .catch(async (e) => {
    console.error(e)
    process.exit(1)
  })
  .finally(async () => await prisma.$disconnect())

3.2. Set up migration script​

Add the migration script to your package.json:

{
  "scripts": {
    "data-migration:add-status-column": "tsx ./prisma/migrations/<migration-timestamp>/data-migration.ts"
  }
}

3.3. Execute migration​

  1. Update your DATABASE_URL to point to the production database
  2. Run the migration script:
npm run data-migration:add-status-column

4. Contract the schema​

4.1. Create cleanup branch​

Create a new branch for removing the old column:

git checkout -b drop-published-column

4.2. Remove old column​

Update your schema to remove the published field:

model Post {
  id      Int      @id @default(autoincrement())
  title   String
  content String?
  status  Status   @default(Unknown)
}

enum Status {
  Draft
  InProgress
  InReview
  Published
}

4.3. Generate cleanup migration​

Create and run the final migration:

npx prisma migrate dev --name drop-published-column

5. Deploy to production​

5.1. Set up deployment​

Add the following command to your CI/CD pipeline:

npx prisma migrate deploy

5.2. Monitor deployment​

Watch for any errors in your logs and monitor your application's behavior after deployment.

Troubleshooting​

Common issues and solutions​

  1. Migration fails due to missing default

    • Ensure you've added a proper default value
    • Check that all existing records can be migrated

  2. Data loss prevention

    • Always backup your database before running migrations
    • Test migrations on a copy of production data first

  3. Transaction rollback

    • If the data migration fails, the transaction will automatically rollback
    • Fix any errors and retry the migration

Next steps​

Now that you've completed your first expand and contract migration, you can:

For more information and updates: